This invention provides a method for improving the storage stability of polymeric articles having an inherent tendency to undergo degradation when exposed to water or a humid atmosphere, probably as a result of hydrolysis. More particularly, the invention is directed to improving the storage stability of articles and devices such as absorbable surgical sutures, clips, staples, implants, prostheses and the like, fabricated from polymers which are susceptible to hydrolytic degradation, notably, polymers and copolymers of glycolic acid (i.e., hydroxyacetic acid), the cyclic dimer of glycolic acid ("glycolide"), lactic acid, the cyclic dimer of lactic acid ("lactide") and related monomers.
Polymers and copolymers of the foregoing kind and absorbable surgical devices made therefrom are well known. See, e.g., U.S. Pat. Nos. 2,668,162; 2,703,316; 2,758,987; 3,225,766; 3,297,033; 3,422,181; 3,531,561; 3,565,869; 3,620,218; 3,626,948; 3,636,956; 3,736,646; 3,772,420; 3,773,919; 3,792,010; 3,797,499; 3,839,297; 3,867,190; 3,878,284; 3,982,543; 4,060,089; 4,137,921; 4,157,437; 4,234,775; 4,237,920; 4,300,565; and 4,523,591; U.K. Patent No. 779,291; D. K. Gilding et al., "Biodegradable polymers for use in surgery--polyglycolic/poly(lactic acid) homo- and co-polymers: 1, Polymer, Volume 20, pages 1459-1464 (1979), and D. F. Williams (ed.), Biocompatibility of Clinical Implant Materials, Vol. II, ch. 9; "Biodegradable Polymers" (1981). The biodegradability of these polymers/copolymers is believed to be due to the hydrolytic attack of their ester linkages by aqueous body fluids although the exact mechanism involved has been a matter of speculation.
An absorbable suture (or other aqueous body fluid-absorbable article) may experience prolonged storage before use, e.g., periods of several months and sometimes even several years. In order to prevent water or humidity in the storage environment from contacting the suture and compromising its in vivo strength to the point where the suture is no longer serviceable, it is common practice to package the suture in an essentially moisture impermeable enclosure. However as noted in U.S. Pat. Nos. 3,728,839 and 4,135,622, any package material which prevents the entry of moisture will also prevent the escape of moisture. Thus, any moisture associated with or absorbed by the suture at the time it is packaged will tend to remain in the package for the entire period of its storage.
According to aforesaid U.S. Pat. Nos. 3,728,839 and 4,135,622, the in-vivo strength of polyglycolic acid surgical elements such as sutures undergoes significant deterioration on long term standing in the package even on exposure of the contents to very small amounts of water for very short periods of time, e.g., 20 minutes or less, just prior to packaging due to the aforenoted tendency of a moisture impervious package to seal the moisture in with the suture.
To prevent hydrolytic degradation of the suture or to minimize its extent, U.S. Pat. Nos. 3,728,839 and 4,135,622 disclose removing moisture from the suture before sealing the package so that no more than about 0.5 percent of water by weight of suture remains in the package once the package is sealed. This approach to improving the suture's storage stability, while effective, is in practice difficult and expensive to carry out. Prior to sealing the suture within its moisture impervious package, it is essential that the suture be "bone dry", a condition achieved by heating the suture for a sufficient period to remove the water therefrom, e.g., 180.degree.-188.degree. for 1 hour under a 26 inch vacuum. However, once the water is removed, the suture cannot be allowed to contact a moisture-containing environment even for a limited duration since as previously noted, even brief exposure to moisture can cause severe deterioration of suture in vivo strength. It therefore becomes necessary following the water removal step to temporarily store the suture in a dry area, i.e., an environment which is essentially free of moisture, where the possibility of contact with moisture is largely eliminated.
Considered in their entirety, these operations for improving the storage stability of absorbable sutures and other surgical devices which are susceptible to hydrolytic degradation amount to a time consuming, expensive and relatively complex solution to the storage stability problem.
In an attempt to overcome the disadvantages associated with storage stabilizing procedures described in U.S Pat. Nos. 3,728,839 and 4,135,622, U.S. patent application Ser. No. 089,735, filed Aug. 26, 1987, the entire contents of which are incorporated herein by reference, discloses a method for improving the storage stability of a polymeric article susceptible to hydrolysis which comprises applying a storage stabilizing amount of at least one water soluble hygroscopic polyhydroxy compound or ester thereof to the polymeric article as a storage stabilizing agent therefor. Such method requires neither a diminution of the article's pre-packaged moisture content nor temporary storage of the article in an artificially-maintained bone dry environment prior to completion of the packaging operation. A problem associated with the afore-described method of said U.S. application Ser. No. 089,735 has been migration of the hygroscopic polyhydroxy compound or ester thereof from the polymeric article to the surrounding packaging material thereby reducing the otherwise desired storage stabilizing effects of the polyhydroxy compound or ester thereof.